Solvation-engineering-enabled quasi-solid positive electrode for four-electron aqueous Zn||Br batteries

Diffusion, crossover, and hydrolysis of bromine species in aqueous electrolytes impede the application of Zn||Br batteries with four-electron 2Br⁺/Br₂/2Br^- conversion. Here, we study a ligand-assisted sparingly solvating electrolyte using ZnSO₄·7H₂O and urea. The high solubility of urea suppresses water activity, while its stronger affinity toward bromine enables preferential coordination over water molecules. This synergistic effect creates a quasi-solvent-free microenvironment that limits bromine solvation and diffusion, resulting in a robust quasi-solid cathode. Moreover, the compact solvation structure favors the formation and stability of Br⁺ species by strengthening nucleophilic urea coordination and inhibiting hydrolysis. Consequently, the fabricated Zn||Br battery exhibits reversible 4e bromine chemistry with a capacity of 417.5 mAh g^-1 at 1 A g^-1 (based on KBr). A zinc battery coupling the 4e iodine positive electrode and 4e bromine (from the electrolyte) is constructed, yielding a total 8e transfer with a capacity of 897.8 mAh g^-1 and a specific energy of 1,344 Wh kg^-1 at 2 A g^-1 (based on KI).